This invention relates to the alignment of visual display systems which use a raster scan for generating the visual scene, and more particularly to the stabilization of the position of the pictorial scene on the screen of the visual system of a modern vehicular simulator such as an aircraft simulator.
Most of the various types of visual systems which are used with modern aircraft simulators use a display system which requires a raster scan similar to that used on commercial TV to generate the desired scene. Such raster scan scene generation is used, for example, in camera model systems such as described in U.S. Pat. Nos. 2,975,671; 3,643,345; 3,670,426 and 3,961,133. Raster scan screen generation is also used in computer generated image systems such as described in U.S. Pat. No. 3,621,214 to Romney et al. Both the camera model systems and computer generated systems typically use a CRT (Cathode Ray Tube) for presentation of the scene. Because of the need for realistic cues, and the practice of using two or more CRT's in modern simulators to create a scene larger than what could be produced on a single CRT, the stability of the scene location with respect to the pilot trainee eyepoint is of the utmost importance. It has been found, for example, that to avoid objectionable cues and distortion that the pictorial representation on the CRT should have such high stability that the long term drift is no greater than 1/2 of a television line width. However, as is well recognized by those skilled in the art, the alignment of the display portion of the CRT screen is affected by a multitude of causes such as variations in the components due to aging, drift due to temperature fluctuations, mechanical settling, etc., and maintaining such stability in the face of so many adverse conditions is extremely difficult. In fact, the drift of direct coupled deflection amplifiers themselves typically used with the CRT's of vehicular visual systems alone usually exceed the criteria of less than 1/2 of a television line width.
The problem of accurately maintaining the picture location on a CRT has been attacked by various feedback systems, such as disclosed in U.S. Pat. No. 2,913,622 issued to J. F. Bartram, et al, on Nov. 17, 1959. This system uses a wire positioned on and attached to the target screen of the cathode ray tube so as to receive electrons from a correctly positioned beam. A signal generated by the electrons received by the wire is gated with a sine wave signal produced by an oscillator. The output of the gating circuit is only that portion of the sine wave which occurs at the moment a signal is generated by the electron beam as it impinges on the wire. Thus, depending on the location of the beam the gate output will either be zero when properly aligned, two positive pulses if too high or two negative pulses if too low. These resulting positive or negative output pulses from the gating circuit are then used to drive and control the supply voltage that provides signals to the deflection plates used to deflect the electron beam. It will be recalled, however, from the above discussion that a very stable scene is necessary in pilot training. Furthermore, it is desirable that the feedback system for stabilizing the pictorial representation on the CRT screen be continuous even during the operation of the system. Therefore, it will be appreciated that the concept and technique described in U.S. Pat. No. 2,913,622 is not suitable for use as a means to stabilize the pictorial scene used in vehicular simulation because of the jitter resulting from the addition of the sinusoidal wave to the deflection plates during the position correcting process.
Other related systems which require information as to the precise location of an electron beam in a CRT include systems having a light detection pen and associated circuitry such as is disclosed in U.S. Pat. No. 3,594,608 issued to Jon C. Mutton on July 20, 1971. It is also necessary in television camera pickup tubes to be able to receive accurate information about the location of the pickup scan which also follows the standard raster sweep. For example, television pickup tubes which are concerned with receiving light in a raster pattern rather than with sweeping an electron beam, are described in U.S. Pat. No. 3,536,824 issued to Klaus Chmillon on Oct. 27, 1970. In this patent, strip light source are located or positioned either on the upper or lower border and on either the left or right border of the screen of a video pickup tube. As a result, when the scanning pickup crosses the light source, a signal is provided that indicates that the beam has traversed to the limit of the screen in that particular direction. This information is then used for alignment.
Still another related technical area which is concerned with the precise location of a scanning energy beam is in the graphic arts. For example, in U.S. Pat. No. 3,609,440 issued to Ronald F. Cooper on Sept. 20, 1971, certain characters are displayed on the face of a cathode ray tube and optically projected onto a film which is supported against a platen. In order that there be minimun distortion, misorientation or skewing of the characters on the film, it is necessary that the base line trace of the cathode ray tube always fall on a particular line or position on the film. In this invention, two unusually shaped apertures are placed or cut in the platen in a location such that a beam of light which passes through the apertures provides either a zero pulse output if the platen is properly aligned, two pulses if the platen is misaligned in one direction, or a single pulse if the platen is misaligned in the opposite direction. Other patents concerned with the alignment of the electron beam of the CRT include U.S. Pat. No. 3,588,584 issued to Matthew P. Tubinis on June 28, 1971; U.S. Pat. No. 3,872,348 issued to Edward T. Astley on June 18, 1973; U.S. Pat. No. 3,720,857 issued to Tetsuji Shimizu, et al on Mar. 13, 1973; U.S. Pat. No. 3,800,072 issued to Jean C. Lejon on Mar. 26, 1974; U.S. Pat. No. 3,358,184 issued to G. G. Vitt on Dec. 12, 1967; and U.S. Pat. No. 3,621,137 issued to Solomon Manber on Nov. 16, 1971. A somewhat related field dealing with the manufacture of semiconductors and micro-circuits is very concerned with the precise location of an electron beam. Accordingly, U.S. Pat. No. 3,832,560 issued to Terence W. O'Keeffe on Aug. 27, 1974 and U.S. Pat. No. 3,811,069 also issued to T. W. O'Keeffe on May 14, 1974 provides examples of circuitry necessary for precise alignment of electron beams.
Even in view of the knowledge that has developed in the area of controlling the position of an electron beam in a CRT, as is demonstrated by the above referenced patents, it will be appreciated that none of these previous systems provides the half TV line width accuracy along with the continuous monitoring and correction capabilities to the extent required and as does the present invention.
Accordingly, it is an object of this invention to provide an apparatus for precisely controlling the position and location of the raster scan on the display screen of a CRT.
It is another object of this invention to provide an apparatus for precisely controlling the location of a raster scan on a CRT on a continuous basis during the operation of the visual system.
Still another object of this invention is to provide apparatus for controlling the location of the raster scan of a display system or visual system other than a CRT system.
A further object of this invention to provide apparatus for precisely locating the raster scan on the display screen of a visual system which is affected by both electronic and mechanical variations.